Field
This disclosure relates to vehicle battery systems, and more specifically to systems and methods for monitoring and controlling high voltage batteries of an electric vehicle.
Description of the Related Art
Electric vehicles generally use one or more electric motors for propulsion and are powered by a battery system. Such vehicles include road and rail vehicles, surface and underwater vessels, electric aircraft, and electronic recreational devices. Electric vehicles release zero air pollutants and generate less noise than conventional combustion engine vehicles. Currently, lithium-ion batteries are often used. Battery performance may vary during use due to battery polarization or decreased charge state. In a typical electric vehicle, the battery management system (BMS) requires the estimation of battery pack power, state-of-charge (SOC), and state-of-health (SOH). These parameters are usually computed with the aid of a battery model.
The equivalent circuit battery model (ECM) is the most commonly seen. This modeling approach leverages the fact that electrochemical battery behaves similar to simple equivalent circuits to mimic battery behavior with common, linear circuit elements. However, despite the superficial similarity of battery and linear equivalent circuits, nonlinear behaviors can be observed in battery that significantly depart from the behavior of linear circuit elements. Two of the best known examples for battery nonlinearity are the Butler-Volmer relationship for charge transfer voltage drop current dependency and the lithium ion diffusion process that manifests itself as an infinite series of RC circuits.
Physics based models, e.g. the Newman model, attempt to capture electrochemical domain knowledge with physical laws and modeling techniques such as finite element analysis or single particle assumption. The physics-based models have the forms to capture nonlinear behaviors in a battery, but obtaining physical parameters such the diffusion coefficient is difficult and requires battery manufacturer's assistance.